The present invention describes Electromachining and more specifically to an NC (Numerically Controlled) arrangement which can be used to machine complex shaped articles such as the airfoils on blisks to a near net shape at a high metal removal rate and with a low tooling cost as compared to turning, milling, EDM (Electro-Discharge Machining) or ECM (Electro-Chemical Machining).
Electromachining is a technique which utilizes the rotating movement of a cylindrical shaped or similar profiled electrode(tapered) about the longitudinal axis with a profiled tip to remove material from a work piece. The kinematics of the machining is analogous to Numerical controlled milling. The electromachining is conducted with a rotating electrode. The tool-electrode is connected to the negative polarity, while the workpiece is connected to the positive polarity of a pulse generator. The pulse on-time may be of sufficiently long periods from hundreds of microseconds to seconds, in some case even long enough to effectively mimic continuous DC current.
A gap between the electrodes allows for the flow of electrolyte. In the electromachining process, it is believed that metal removal is a result of intense, controlled electroerosion.
The electrical erosion can be created by electrical breakdown of the electrolyte and vapor-gas layer produced at the interface between the tool and the workpiece where material is being removed, as well as by instantaneous short-circuits or transient arcs between the electrode and workpiece. Due to electrical erosions on the machining surface, a large number of pits or craters are formed.
Conventional milling is widely used to rough machine blisks (bladed discs). Subsequent machining processes such as ECM may be used for finish machining. For blisks made of difficult to machine alloys such as IN718, the milling process typically has long cycle times, high cutting tool cost, and high equipment cost. Additionally, long tool extensions necessary to reach into the machined pockets coupled with large cutting forces place limits on the rate and accuracy of material removal.
A great deal of effort has been expended in connection with high speed milling techniques that utilize high spindle rotation speed, improved cutting tools, and extremely rigid machines. High speed milling adds significant equipment and tooling costs but has not shown great success in machining blisks made of tough nickel alloys. One reason for the difficulty is the mechanical limitations imposed by a mechanical cutting process. That is to say, a cutting tool having a moderate to high slenderness ratio can only tolerate a small load before tool deflection impacts accuracy of the milled product. Additional loading may break the cutter. Tool deflection also alters the tool's cutting geometry relative to the workpiece, leading to less than optimum performance in terms of tool life and accuracy of the milled product. Mechanical loading is also limited by part deflection and desired accuracies.
There is a need to explore non-mechanical milling processes. Traditional NC EDM milling is a very slow process and it cannot meet the industrial need for high speed machining. This invention aims to achieve high speed through enhanced electroerosion.